Solid-state lamps are replacing fluorescent and incandescent lamps due to their superior efficiency. The lighting element in a solid state lamp comprises semiconductor material where light is emitted by electroluminescence. Various types of lighting element may be employed such as light emitting diodes (LEDs) of various types (including silicon, organic and polymer LEDs). The present disclosure will discuss in particular the case of an LED but it will be appreciated that it applies in general to other types of solid-state lamps.
The current through a solid state lighting element needs to be controlled to ensure that a minimum amount of power is provided so that light of the required brightness is emitted, but also that a maximum amount of power is not exceeded in order to prevent damage to the element and the circuit. To take care of this, an LED driver circuit is provided.
Ballasts are also often used in lighting systems. A ballast is a device or circuit which limits the amount of current supplied to a load. They are commonly used in devices which exhibit a negative resistance characteristic such as gas discharge lamps, where limiting the current is important to prevent the lamp being destroyed or failing. However, ballasts are also useful for limiting the current in ordinary positive resistance circuits, including for use with solid state lamps. The ballast is usually integrated with a luminaire housing, for coupling with the driver circuit of a solid state lamp via suitable electrical connectors when the solid state lamp is inserted into a socket of a luminaire housing.
FIG. 1 illustrates an LED lighting system 100 where an LED driver circuit comprises a linear regulator 102, which supplies power to a series of LEDs 104 shown generally as LED1 through LEDn. The linear regulator 102 is coupled with the mains supply 106 provided by an electrical grid (or alternatively by an off grid power supply). A linear regulator can be connected to a mains AC supply, but has a low efficiency and high power loss, leading to high heat dissipation. When connected to a ballast, the linear regulation needs to be adjusted in order to balance the input and output power of the LED driver circuit.
FIG. 2 illustrates a lighting system 200 in which a switching regulator 202 is used to control the power supplied to the LEDs 104. The switching regulator has a switching element that acts to selectively charge or discharge an energy storage element and to selectively couple the energy storage element with the load, to regulate voltage or current characteristics and control the power supplied to the load. Various different circuit topologies are known, including without limitation a buck converter, a boost converter, a buck-boost, and a single ended primary inductor converter (SEPIC). A switching regulator 202 is more energy efficient than a linear regulator.
A switching regulator 202 works well with an AC mains power supply or with a magnetic ballast, which operates at a similar frequency. However, when directly connected to an electronic ballast, the switching regulator 202 can cause flickering, incorrect output voltage and current, unstable and unreliable operation, or even damage to the circuit. This is because switching regulators are designed to work with an AC input or a magnetic ballast, both of which are voltage sources. However, an electronic ballast acts as a current source and so when connected to a ballast the input energy and further the input voltage of the switching regulator 202 depends on its output part. If there is any mismatch between the input power and the output power, the switching regulator 202 will see either insufficient input voltage or excessive input voltage.
Therefore it can be understood that existing LED driver circuits are designed to be used either exclusively with a mains supply or with an electronic ballast. If an LED driver circuit is designed for use with a mains supply it will not function properly with a ballast, and vice versa.
This creates a problem for lamp interoperability, and for retrofitting of lamps. A solid state lamp is usually provided in a bulb housing and includes the driver circuit and the light emitting element. A lighting system may comprise a luminaire which has a housing and a socket for receiving a lamp. A ballast will normally be provided as part of the luminaire for electrical connection with a lamp which is inserted, via appropriate mating electrical connectors provided by the bulb housing and the luminaire socket.
A major concern with installing and maintaining lighting installations is interoperability of lamps with different types of ballasts. A luminaire may or may not have a ballast fitted, and the driver circuit of a solid state lamp may or may not be compatible with a ballast. So this means the compatibility of the lamps and light fixtures must be checked carefully. This increases inconvenience for users, and introduces complexity for manufacturers who must manufacture, administer and support multiple different products to account for the various combinations that are possible.